1. Field of the Invention
The present invention relates in general to surface acoustic wave devices and in particular to an acoustic wave device in which the propagation path of the surface acoustic wave device is folded in order to make better use of the total chip length.
2. Description of Related Art
It is well known in the surface acoustic wave art that in order to get a narrower bandwidth, the longer the surface acoustic wave device must be. Of course, the longer the device is, the larger the chip size that is required and thus there is less efficient use of circuit space. It has been known to use first and second transversely-spaced transducers separated by a first multistrip complex (MSC) with a second MSC longitudinally spaced from and overlapping the first transducer and a portion of the first MSC. A third MSC is also longitudinally spaced from and overlaps the remaining portion of the first MSC and the second transducer. A wave propagated by the first transducer is reflected by the second MSC to the first MSC and from the first MSC back to the third MSC. The third MSC again reflects the signal back to the output transducer. Thus, the propagation path of the surface acoustic wave signal has been lengthened four times. The disadvantage with this device is that the reflecting MSC's are lossy and thus the entire filter is a high-loss device. Further, because the travel path has to pass through three multistrip coupling devices, signal distortion occurs.
Other filters include input and output transducers that are both transversely spaced and longitudinally spaced with respect to each other and with two slightly inclined reflectors therebetween to form a Z-shaped filter. Thus the signal from the first transducer travels longitudinally until it encounters the first inclined reflector which reflects the signal at an inclined angle in the reverse direction where it encounters the second Z-shaped reflector. The signal is again reflected longitudinally to the output transducer. This device again is a lossy device because of the reflector gratings. Further, because of the inclined angle of the reflector gratings, increased signal distortion occurs.
It would be extremely important for a surface acoustic wave (SAW) device to be made such that the propagation path is folded in order to produce an output signal that is equivalent to that generated by a much longer surface acoustic wave device. Such device, if it could be made with low loss and low distortion, would conserve chip space and enable the use of much smaller chip sizes.
French Patent No. 2,685,580 discloses a double-ended version of such a SAW device in which the propagation path is folded. As disclosed therein, N independent in-line surface acoustic wave (SAW) devices are utilized where N.gtoreq.3 and is only an odd integer. Each device again includes at least an input transduction means and an output transduction means. A common input signal terminal is coupled to each of the input transduction means and a common output terminal is coupled to each of the output transduction means. One of the in-line SAW devices has a cavity between the input and output transduction means with a length of X.lambda. where X is an arbitrary number. The cavity in each of the N-1 remaining in-line SAW devices has a length of ##EQU1## where M is an arbitrary constant including zero and n=the individual track number. This allows a differential time delay, .DELTA.t, to occur between the application of an input signal to the common input terminal and the detection of an output signal at the common output terminal to be substantially N times the corresponding delay of the first cavity alone.